The present invention relates to switches having bifilar-fashioned superconducting routes for controlling the flow of current through superconductors, and utilizes the change between the superconducting state and the nonsuperconducting state of the superconducting routes as the switching mechanism. These switches may preferably be used in conjunction with superconductive coils for magnetic levitating trains, magnetic resonance imaging apparatuses, and the like.
Electrical switches such as those described above are conventionally utilized in conjunction with superconductive coils used for magnetic levitating trains, magnetic resonance imaging apparatuses, and the like, in order to deliver a constant flow of electric current over a long period.
Most superconductive switches are categorized into one of two types. One type utilizes mechanical movement for the opening and closing of the switches, such as by contact and breaking of contact of contact points. The other type is a temperature-dependent type of switch utilizing changes of conducting state of superconductive wires by means of temperature change, i.e., the temperature-dependent type superconductive switches function by switching between the normal conductive condition (resistant to the flow of electrical current), and the superconductive condition (not resistant to the flow of electrical current) of the superconductive wires so as to open and close the switches. At present, by virtue of light weight (and thus ease of handling) and since the conducting resistance is comparatively low when the switch is in the closed condition, the majority of superconducting switches employed are of the temperature-dependent type.
In order to produce light and compact superconductive switches of the temperature-dependent type, and in order to enhance the current density, the superconductive wires are wound in such a manner that electromagnetic induction in the superconductive wires is minimized. In such structures, the magnetic fields generated by the superconductive switches themselves are reduced, and the critical current density which can flow through the superconductive wires is increased. More specifically, in the temperature-dependent type of superconductive switches of the prior art, as shown in FIGS. 1 and 2, superconductive wire 1 is wound around a cylindrical core 2 in a bifilar fashion. Such a superconductive switch is disclosed in Japanese Patent Application Publication (B2) No. 56-26993. In this case, two superconductive routes 1a and 1b terminate at terminals 1c, and the terminal 1c of one superconductive route is unitary with the terminal 1c of the other superconductive route, thereby constituting one superconductive wire 1. These superconductive routes 1a and 1b are wound around the core 2 at a uniform pitch in such a manner that the two superconductive routes 1 a and 1b are in contact with each other over the entirety of the winding. The superconductive routes 1a and 1b are connected to the positive and the negative terminals of an external device, respectively, so that the electric current flows from route 1a to route 1b. The switching-off operation of the superconductive switch is accomplished by activating heater line 3 which heats the superconductive routes 1a and 1b; whereas the switching-on operation is achieved by means of cooling the routes 1a and 1b with liquid helium or other coolant, depending on the critical temperature of the superconductive material used in routes 1a and 1b. In FIG. 1, a receiving material 4 for receiving liquid helium, and a casing 5 for protecting the internal structure are shownby dotted lines (illustration of the heater line 3, receiving material 4, and casing 5 is omitted from FIG. 2).
Conventionally, for the material of the superconductive wire 1, a Nb-Ti alloy is selected for the superconductive material in order that a high density of electric current flow is assured when the superconductive switch is ON. In this case, the Nb-Ti alloy is formed into a large number of ultrafine superconductive filaments having diameters of a few .mu.m, and the filaments are arranged lengthwise in the superconductive wire 1. On the other hand, a relatively electrically resistive material, for example, a Cu-Ni alloy, is selected for the matrix of the superconductive wire 1, which substantially insulates respective superconductive filaments from one another, in order to ensure high resistance when the switch is OFF. However, such relatively resistive material does not contribute to stabilizing the superconducting properties of the superconductive wire relative to the conductive material, e.g., Al or Cu. Thus, an undesirable instability in the aforementioned superconductive wire may occur in which the probability is high that the state may change from the superconducting state to the normal conductive state (resistive state) when electric current flows through the wire. That is, if a relatively electrically-resistive material is used as the matrix of the superconductive wire 1, it is not possible for an excess of current to escape to the matrix from the internal superconductive filaments inside of the wire 1. Therefore, a portion of the internal superconductive filaments may change to the normal state even if a small unexpected heat disturbance occurs, and additional heat may be generated by this state change. The heat may then cause the propagation of this effect throughout the entire superconductive wire 1, turning the superconductive switch into the OFF condition at an unexpected time.
If such a wire includes a large number of superconductive filaments, and thus, has a critical current much larger than the normal operating current, the incidence of the aforementioned undesirable phenomenon can be decreased. However, if the number of superconductive filaments is increased, a corresponding increase in the cost, mass, and size of the superconductive switch may be required.
Therefore, in order to produce a light and compact superconductive switch at a reasonable price, while ensuring that the superconductive switch does not change from the non-resistive condition to the resistive condition at an unexpected time, such heat disturbances should be prevented. It is suspected that such heat disturbances are primarily caused by small amounts of heat generated by small electromagnetically-induced movements of the wire (in the range of a few .mu.m). Conventionally, attempts have been made to restrain this kind of wire-movement by setting the wires in a resin material, such as epoxy-resin.
At present, these superconductive switches are sometimes utilized in strong magnetic fields, especially when the switches are connected to superconductive coils for magnetic levitating trains or for magnetic resonance imaging apparatuses. For example, as illustrated by arrow A in FIG. 2, the superconductive switch receives magnetic force along the axial direction thereof. In this case, once the current flows into the superconductive switch, the route 1a, which conducts the electric current, is depressed radially inwardly with respect to the core 2 according to Fleming's left-hand rule. At the same time, the route 1b, which returns the electric current, is forced radially outwardly and away from the core 2 in directions opposite to that of the route 1a. In other words, in the conventional superconductive switch in which the routes 1a and 1b are simply wound in a bifilar fashion, wire-movement is likely to occur due to the above-described opposing electromagnetic forces.
As mentioned above, if the superconductive wires are impregnated in a resin material, and if the opposing electromagnetic forces are small, the superconductive wire may be held immobile by virtue of the adhesive force of the resin material overcoming the electromagnetic forces, thus restraining the movement of the wire. However, the adhesive force of the resin material is limited. If the electromagnetic forces are large, the wire-movement may not be prevented. In particular, if the switch is cooled to -269.degree. C. (4.2K, the boiling point of liquid helium) after the wire was impregnated in the resin material at room temperature, the routes 1a and 1b will likely separate from each other because of a difference in the thermal-shrinkage ratios of the superconductive wire and the resin material. Once wire-movement commences, heat is generated by the spiral current in the matrix of the wire and by the friction between the adjoining superconductive routes, and in addition, heat is generated due to release of stress when the superconductive wire separates from the resin material, and also due to friction between the wire and the resin materials. As a result, even if the superconductive wire is impregnated in a resin material, current flow may be unstable due to heart generated by one or more of the above mechanisms. In this case, the superconductive switch will change to the normal conductive state (resistive state) at a current level less than the half of the critical current.